1. Technical Field
The present invention relates generally to mixtures of certain polymers and photoacid generators used in resist compositions and specifically to the photosensitization of aromatic iodonium compounds for visible light or laser direct imageable photoresists.
2. Description Of The Prior Art
Fabrication of printed circuit boards by Laser Direct Imaging (LDI) offers a number of advantages over conventional exposure technologies. These include reduced defects, faster engineering changes of circuit design and material and labor savings resulting from the elimination of photomasks. The current generation of direct imaging tools rely upon argon ion lasers which emit in the visible region of the spectrum. The practical application of LDI technology requires the development of new resist systems since conventional ultraviolet sensitive photoresists are virtually inactive at longer wavelengths.
In pursuit of these objectives, several resist vendors have modified their dry film acrylate resists by the addition of sensitizers active at visible wavelengths. This approach has met with a number of problems in trying to fulfill todays technological requirements and does not appear capable of meeting future needs. The presently available resists are prohibitively expensive for many applications, i.e., 4-6 times the cost of corresponding UV resists in some cases, with little prospect of significant price reductions in the future Furthermore, the use of low energy (long wavelength) sensitizers often results in a decreased thermal stability and shelf life of the resist Generally these systems exhibit a strong oxygen sensitivity which precludes their use in high resolution applications. Additionally, the problem of oxygen inhibition makes the use of liquid apply methodology impractical since the oxygen diffusion barrier normally provided by the dry film cover sheet would be absent.
An alternative approach which overcomes many of the above noted problems inherent in radical cross-linking systems involves the use of resists based on acid catalyzed chemistries (e.g. polymerization, depolymerization, side chain cleavage, etc.). Such systems are described in, e.g., In Polymers in Electronics, Davidson T. Ed., ACS Symposium Series 242, American Chemical Society, Washington, D.C., 1984, p.11, H. Ito, C. G. Willson. Systems of this type are capable of high resolution, i.e., submicron in the case of semiconductor applications. Since they are oxygen insensitive they can be applied in liquid form as thin films. This is an important advantage, providing the potential for better resolution, lower cost and improved adhesion.
Despite these advantages, the development of such resists for LDI applications requiring high photospeeds has been impeded by the absence of an efficient photosensitizer/photoinitiator system. The use of onium (e.g. triphenylsulfonium and diphenyliodonium) salts as photoinitiators for photoacid generation is well known and is described by S. P. Pappas in J. Imaging Technol., 11, 146 (1985). The photosensitization at ultraviolet, and to a more limited extent, visible wavelengths has been reported, Crivello et al, J. Polym. Sci. Polym. Chem. Ed., 16, 2441 (1978); Crivello et al, ibid., 17, 1059 (1979); Saeva, ACS PMSE Preprint, 61, 72 (1989); U.S. Pat. No. 4,250,053 (Smith); U.S. Pat. No. 3,729,313 (Smith); and U.S. Pat. No. 4,069,054 (Smith), although efficient sensitization becomes increasingly difficult at longer wavelengths. The current initiator systems known to us have a number of disadvantages which make them unsuitable in acid sensitive resists. Most of the visible sensitizers reported for diphenyliodonium salts bear amino substituents and are used in free radical or epoxy systems wherein the presence of such basic functionality does not pose a problem and where low solubility is not as great a concern since high sensitizer concentrations are not generally required. Other argon laser cationic crosslinking systems have been reported but they suffer from shelf-life problems and are hindered by an inability to harvest all of the laser light, Ichimura et al, J. Applied Polymer Sci., 34, 2747 (1987). In short, none of the presently available systems have the qualities required of a good sensitizer for acid catalyzed systems for laser direct imaging.
A need exists, therefore, for a photosensitizer/initiator system having the required solubility and other properties necessary for practical use and whose photochemical and photophysical parameters have been tailored to the output of the argon ion laser while maintaining functional compatability with acid catalyzed processes. Such a system will allow the design and development of photoresists based on acid catalyzed chemistry for laser direct imaging applications.